The selection of a suitable site always play a major role in shrimp farming. The determination of a site for shrimp farming is made only after through analysis of information on topography, ecosystem, meteorological and socioeconomic conditions in relation to farm design, species compatibility and economic viability. Criteria are herein presented that could serve as guidelines in judging site suitability.
Fig. 4. Pond layout for intensive shrimp farming
(earthen pond with Concrete dikes).
|CROSS SECTION OF CIRCULAR TANK||A: Gate value of spray pipe|
|B: Aeration bubbling tube|
|C: Sand bed|
|D: Cylindrical screen|
|E: Spray pipe|
|F: Gate value of supplying pipe|
|G: Drain pipe|
Fig. 5. Shigueno type intensive culture tank.
Water quality includes all the inherent physico-chemical and microbiological characteristics of water. pH is generally considered as one of the most important factors. In any chosen site, the pH of the water preferably range from 7.5 to 8.5. The other equally important chemical characteristic of water is the level of oxygen saturation throughout the water column. Fluctuations in dissolved oxygen level should be predetermined and the oxygen level is preferably not lower than 4 ppm.
The water must not be too turbid. Water with very heavy silt load can cause siltation problems in the water supply system, eg., clogging of filter nets or net enclosures and increasing sedimentation at the pond bottom. The water is preferably to be rich in microorganisms.
Salinity variation is considered a determinant factor in shrimp production. Optimal level varies from species to species. For instance, the tiger shrimp (Penaeus monodon) grows faster at 15–30 ppt. The white shrimp (P. indicus and P. merguiensis) tolerate higher salinity ranges (25–40 ppt). Ideally, salinity should remain uniform at normal weather and should not drop abruptly during rainy days.
The tidal characteristics of the proposed site should be known. Knowledge of this parameter is of extreme importance in determining pond bottom elevation of dike, slope ratio and drainage system.
Areas best suited for shrimp farming should have moderate tidal fluctuations preferably 2–3 meters. In areas where the tidal range is greater than 4 meters, the site may prove uneconomical to develop or operate as large and high pond dikes will be required. In areas where tidal range is less than one meter, water management will be expensive requiring the use of pumps.
A salient point to consider in relation to tidal range is the knowledge of the occurence of highest high and lowest low water levels. This should be known so that the size and height of the perimeter dike can prevent flooding. In addition, direction and strength of water current should be known for provisions on dikes construction to reduce erosion.
Lastly, the proposed area must not be adversely affected by any industrial or agricultural pollution.
The types and texture of the soil of the area should be analyzed before settling on a site for shrimp farming. Soil samples must be taken at random location, preferably up to a depth of 0.5 meter and subjected to physical and chemical tests to determine the acidity, amount of organic load, level of fertility and physical composition.
The soil at the proposed site should have enough clay contest. This is to ensure that the ponds constructed will hold water. Good quality dikes are usually built from sandy clay or sandy loam materials which harden and easily compacted. The dikes will not crack in dry weather. Clay-loam or silty-clay loam at pond bottom promotes growth of natural food organisms. Diking materials made of undecomposed plant matter and alluvial sediments should be avoided.
Most ponds developed along the coastal areas with dense mangrove vegetation often have acid-sulphate problem during the first few years of operation. This is due to the accumulation of pyrites (iron sulfide) in coastal soil. Breakdown of pyrites is minimal in submerged soil.
During pond construction, the subsoils are dug for dikes and the pond bottom levelled, the pyrites become oxidized producing sulphuric acid which acidifies the soil. The pH of water becomes extremely low and hence affecting water quality and correspondingly reduced natural productivity.
Alleviating acid sulphate conditions in ponds requires the use of lime and removal of acid by leaching and flushing.
It is essential to have a detail topography of the selected site for pond design and farm layout. Coastal sites where the slopes run gently towards the sea are easier for pond development requiring less financial inputs since excavation is minimal. Filling and draining of water likewise is easily facilitated by gravity.
In areas where the above conditions are not available, the use of mechanical pumps may be resorted. Associated with topography related constraints are the availability of sufficient quantity of soil for dike construction obtained from excavation of ponds or from above ground bunds. It may prove uneconomical if the site to be developed would require diking material to be transported from outside the chosen area.
The type of vegetation in the area can be, to some extend, indicative of physical elevation and soil type. Dominance of the mangrove plants Avicennia spp. is an indication of good and productive soil. Outgrowths of Rhizophora spp. which are usually characterized by dense prop root systems usually signifies soil types that are coarse and acidic.
It is of primary importance to consider density of shrubs at the site. These have to be cleared first before any land development can take place. Clearing operation can add up to the capital outlay.
Close proximity of the site to the fry ground is advantageous in that the animals being collected for stocking are not subjected to too much transport and handling stress.
Accessibility is an important consideration in site selection. Overhead cost and delay in the transport of materials and products can be minimized.
Adequate consideration should be given to a number of farm related matters such as the availability and quality of labor, peace and order situation or the area concerned, availability and source of electricity and water supply, marketing channels and facilities. The availability of technical assistance near the site is another advantage if a site is chosen near an aquaculture research institution.